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This crustose epiphyte is one of the ‘scripture lichens’. The thallus is faint, grey or brown and the discs (lirellae) may open almost to their limit. The variably sized discs are frequently covered over with a yellow or grey dusting of pruina. It is a common species of older, shaded trees with dry, basic bark (such as ash and sycamore). It is frequent in suitable habitats throughout Ireland.

Key characteristics

On dry basic bark; discs often fully open with a yellow or grey dusting of pruina

Under the microscope, spores are four-six-septate (three-septate in O. herbarum, which has less opened discs and lacks pruina).